In order to obtain a desired all-pass modulation characteristic of a phase-locked loop (PLL) of a radio transceiver, in which the transfer functions for two ports of the PLL are complementary, both ports are modulated simultaneously and the overall transfer function seen by the modulating signal will preferably have an all-pass characteristic. In order to achieve the desired all-pass characteristic, however, both PLL ports must have equal gains. While the digital gain is stable and accurate, the analog gain can vary with process and temperature and must be trimmed in the factory to equal that of the digital gain.
A factory test station, sometime referred to as a trim station, may include test equipment that can receive the modulated carrier signal and measure its modulation accuracy. The trim procedure would include transmitting a known modulation waveform from the radio transceiver, while varying the analog gain until desired modulation accuracy is achieved. The need for an external receiver adds to the cost and complexity of the trim station.